1. Field of Invention
The present invention relates in general to a fiber-distributed strain and temperature measurement based on the Brillouin scattering effect, more particular, to a method of utilizing a dispersion-shifted fiber for simultaneously measuring the distributed strain and temperature through Brillouin frequency shift.
2. Description of Related Art
There are two ways to fulfill the distributed sensing approach. One includes the use of single sensors being discretely arranged along a sensing line, but will make the whole sensing system much complicated. The other one as described hereinafter includes the use of optical-fiber sensors to obtain the detecting physical parameters along a linear fiber depending upon the optical characteristics thereof. Under the circumstances, the optical fiber are regarded, on one hand, as an active component for sensing measurement and, on the other hand, as a passive component for the information transmitting material to obtain the following advantages:
1. The optical fiber is small in volume and light. Thus, the optical fiber can be adopted easily anywhere.
2. Since the frequency bandwidth of the optical fiber is large, many signals may be transmitted simultaneously.
3. Since the optical fiber is made of nonconductive insulating material, it is not influenced by external electromagnetic waves. Thus, the signal may be transmitted without noise.
4. Due to the development of optical fiber technology, optical fibers can be manufactured at a low cost.
As such, the utilization of fiber-distributed sensing for the measurement of strain and/or temperature distribution is widely applied on many implementations to monitor such as tunnels, bridges, dams and airplanes, buildings and etc. for safety-secured purpose.
Recently, the fiber-distributed strain/temperature measurement based on the Brillouin scattering effect has generated significant interest in the technical community. The Brillouin frequency shift is dependent on the temperature and strain conditions of the optical fiber, which provides the basis for a sensing technique capable of detecting these two parameters. The challenge, then, is to develop a technique to distinguish their individual contributions to the change in the Brillouin frequency of the optical fiber and to avoid the cross-sensitivity problem. Several conventional techniques have been reported to accomplish the simultaneous measurement of fiber strain and temperature. In these methods, the one disclosed by X. Bao, K. J. Webb, and D. A. Jackson, published in Opt. Lett., 18, 141 (1993), that uses a half of the fiber isolated from the effects of strain for temperature sensing, and another one disclosed by M. A. Davis and A. D. Kersey published in IEE Proc.-Optoelectronic., 144, 151 (1997), that utilizes the fiber Bragg gratings combined with optical fiber, both require rather complicated sensing structure. Another technique disclosed by T. R. Parker, M. F. Farhadlroushan, V. A. Handerek, and A. J. Rogers, published in IEEE Photon, Technol. Lett., 9, 979 (1997), that simultaneously measures the Brillouin power and frequency shift can achieve the fully distributed measurement of strain and temperature, but the temperature resolution is limited by the power measurement accuracy (xcx9c0.013 dB for 1xc2x0 C.), which is very difficult to reach 0.05 dB over a considerable length of fiber and is very sensitive to environment.
It is therefore, in one aspect, an object of the present invention to provide a method that utilizes a dispersion-shifted fiber having compound compositions with different temperature coefficients in core as the sensing fiber to measure the distributed strain and temperature simultaneously. It needs only the measurement of Brillouin frequency shifts of the Brillouin spectra and can accomplish the high resolution and accuracy of temperature and strain measurement without modifying the sensing fiber.
In another aspect of the invention, it is an object to provide a method for simultaneously measuring the distributed strain and temperature without complicate detecting and calculating procedures so as to simplify the sensing system with good result of effectively distinguishing their individual contributions to the change in the Brillouin frequency of the optical fiber and successfully avoiding the cross-sensitivity problem.
According to the above-mentioned objects of the present invention, the method for simultaneous measurement of distributed strain and temperature, includes the steps of (a) providing a dispersion-shifted fiber having compound compositions with different temperature coefficients in core, (b) obtaining a multi-peak Brillouin spectrum of the dispersion-shifted fiber, (c) determining two peaks of the Brillouin spectrum having a first strain coefficient and a first temperature coefficient at a first peak frequency and a second strain coefficient and a second temperature coefficient at a second peak frequency, wherein the first strain coefficient is equal to the second strain coefficient, a Brillouin frequency shift of the peak relating to strain and temperature of the dispersion-shifted fiber, (d) calculating a temperature change from dividing a variation of the Brillouin frequency shifts of the two peaks by a variation of the first and second temperature coefficients, and (e) calculating a strain change by substituting the known temperature change into the equation of Brillouin frequency shift of the peak relating to strain and temperature of the fiber.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.